Human Physiology, 14th edition (2016)

372 Chapter 12

action potentials. The stage is now set to explain exactly how a

motor neuron stimulates a muscle fiber to contract.

The release of acetylcholine from axon terminals at the

neuromuscular junctions (motor end plates), as previously

described, causes electrical activation of skeletal muscle fibers.

End-plate potentials (analogous to the EPSPs described in chap-

ter 7) are produced that generate action potentials when they

reach a threshold level of depolarization. Action potentials in

muscle cells, like those in nerve cells, are all-or-none events that

are regenerated along the plasma membrane. This is because

voltage-gated channels that produce action potentials are located

immediately adjacent to the motor end plates and all along the

sarcolemma and transverse tubules. It must be remembered that

action potentials involve the flow of ions between the extracel-

lular and intracellular environments across a plasma membrane

that separates these two compartments. In muscle cells, there-

fore, action potentials can be conducted into the interior of the

fiber across the membrane of the transverse tubules.

The transverse tubules contain voltage-gated calcium

channels, also called dihydropyridine (DHP) receptors (after

a class of drugs that specifically bind to and block these chan-

nels). These respond to membrane depolarization. When the

Figure 12.16
Excitation-contraction
coupling in skeletal
muscle. (1) ACh released by
somatic motor neurons binds
to nicotinic ACh receptors in
the sarcolemma, causing a
depolarization that stimulates
(2) voltage-gated channels,
producing action potentials.
(3) The conduction of
action potentials along the
transverse tubules stimulates
the opening of voltage-gated
Ca^2 1 channels. (4) These
channels in the transverse
tubules are mechanically
coupled to Ca^2 1 release
channels in the sarcoplasmic
reticulum, causing them to
open. Ca^2 1 then diffuses
out of the sarcoplasmic
reticulum, so that it can bind
to troponin and stimulate
muscle contraction.

SarcoplasmicSarcoplasmic

reticulumreticulum

Axon terminal

Ca2+

Transverse tubule voltage-gated

calcium channel

Sarcoplasmic reticulum calcium

release channel

Nicotinic acetylcholine receptor

Skeletal muscle voltage-gated

sodium channel

TransverseTransverse

tubuletubule

Transverse

tubule

1

2

3

4

1

2

3

4

Sarcoplasmic

reticulum

Sarcolemma

Figure 12.15 The sarcoplasmic reticulum. This
figure depicts the relationship between myofibrils, the transverse
tubules, and the sarcoplasmic reticulum. The sarcoplasmic
reticulum ( green ) stores Ca^2 1 and is stimulated to release it by
action potentials arriving in the transverse tubules.

Sarcoplasmic
reticulum

Sarcolemma

Myofibrils

A band

I band

Z lines

Nucleus

Terminal
cisternae

Transverse tubule

Mitochondria